1. Field of the invention
This invention relates to an image signal processing method for storing a series of image signals constituting an image on a recording medium such as an optical disk or transmitting the image signals to a remote section, and reproducing the image by use of the stored or transmitted image signals.
2. Description of the Prior Art
In recent years, an electronic filing system has been developed and put into practice. In the electronic filing system, an image recorded on a recording sheet is read out as digital image signals, appropriate image processing is conducted on the digital image signals, and the processed digital image signals are stored on a recording medium such as an optical disk and read from the recording medium when necessary for use in reproduction of a visible image.
In general, in the aforesaid electronic filing system, the image is read out by scanning of the recording sheet carrying the image recorded thereon, and the image signals obtained by the scanning are stored on the recording medium in the array sequence of scanning lines. Specifically, the image signals generated by sequential scanning of the image are stored on the recording medium in the sequence of generation of the image signals.
By way of example, an optical disk filing system wherein the image signals are recorded on an optical disk will hereinbelow be described in more detail. FIG. 4 is a schematic view showing the array sequence of scanning lines on a recording sheet, FIG. 5 is a schematic view showing the storing condition of image signals on an optical disk, and FIG. 6 is an enlarged view showing a part of the optical disk of FIG. 5. As shown in FIG. 4, a recording sheet 10 carrying an image recorded thereon is scanned in the sequence of a first scanning line L1, a second scanning line L2, a third scanning line L3, . . . from the upper portion of the recording sheet 10. As shown in FIG. 5, image signals generated (read out) by the scanning are stored in a spiral form on an optical disk 12 in the sequence of generation of the image signals. Therefore, the image signals are continuously stored on a track 14 in the array sequence of scanning lines, i.e. in the sequence of the first scanning line L1, the second scanning line L2, the third scanning line L3, . . . .
However, with the optical disk, a burst-like image signal error may arise when the image signals are stored on the optical disk or read out therefrom. For the purpose of this application, the term "burst-like error" is defined to mean such an error that a group of the image signals cannot be read out or, though they can be read out, the readout signals are not correct and not reliable. For example, as shown in FIG. 6, in the case of an optical disk 12, a single track 14 is ordinarily divided into a plurality of sectors (e.g. 64 sectors), and storing and reading of the image signals are conducted in an unit of the sector. Therefore, an error caused in the course of the storing, reading, or the like of the image signals often arises in a unit of the sector. In FIG. 6, the hatched portion denotes a single sector.
However, the image signals of, for example, three or four scanning lines are stored at a single sector in the array sequence of the scanning lines. Therefore, when a burst-like error in a unit of the sector arises, a group of the image signals on three or four adjacent scanning lines, for example, on the fourth scanning line L4, the fifth scanning line L5 and the sixth scanning line L6, become unreproducible at the time of image reproduction. Thus a serious image defect arises.
Though the problem that an image defect is caused by a burst error is typical in the case of the optical disk, it may arise also in the case of other recording media, for example, a magnetic disk, a magnetic tape, and a floppy disk.
The aforesaid problem arises not only in the course of storing the image signals on the recording medium and image reproduction but also in the course of transmitting the image signals with a facsimile device or the like.
For example, as one method of digital communication, packet communication wherein a series of information data is divided into units (called packets) and transmitted in this form is known. When the image signals are to be transmitted by packet communication and an error arises in one packet among a series of packets, a burst-like error is caused in the image signals corresponding to the error portion in the packets, and a group of the image signals becomes unreproducible.